As with any rapidly rotating power source and an apparatus driven thereby, it is desirable to be able to selectively make and brake the driving connection quickly and remotely, and to transfer torque between the two smoothly, quietly and efficiently. A good example is a typical automotive engine driven air conditioning compressor. While some newer compressors are either electrically driven directly, or of a clutchless variable capacity design, many if not most are still engine driven, and thus require a clutch.
Such a clutch is typically electromagnetically activated, in response to a signal indicating air conditioning demand, by a coil assembly. The coil is mounted stationary to the front of the compressor housing, and is axially opposed to a pulley that rotates freely on a bearing located on the compressor housing and which carries an annular friction disk. The pulley is spun by an engine driven belt, but is axially stationary. The compressor drive shaft extends axially out of the compressor housing and through the pulley assembly with radial clearance. A central hub is fixed to the end of the shaft, and an integral or fixed drive plate radiates out from the hub, typically, though not necessarily, with three arms or lobes. An annular armature plate of magnetic material, typically low carbon steel, is axially opposed to the friction disk of the pulley, and, when pulled into firm axial engagement therewith by the magnetic attraction of the activated coil behind the pulley, spins with the pulley one-to one.
In order to transfer torque from the spinning armature plate to the drive plate and compressor shaft, an additional mechanism is necessary to mount the armature plate to the drive plate in its engagement ready position. and to return it thereto when de activated. Such a mechanism, ideally, will also serve to dampen the noise of plate to pulley engagement and disengagement. Another issue is torsional vibrations that can be created within the compressor as it pumps. Two main compressor torsional vibrations are of concern. One is the primary pumping order of the compressor, dependent upon its number of pistons, which exists at all compressor speeds. The second is compressor's own shaft resonance. Either or both of these may resonate with a vehicle's own operating frequency, creating excessive noise and vibration. A means associated with the clutch to dampen torsional vibrations is therefore also desirable.
An old and basic armature mounting mechanism that provides torque transfer and automatic decoupling, but no significant vibration dampening function, consists of just a plurality of leaf springs, typically three, riveted at each end between the annular armature plate and the drive plate. The springs, in a free state, hold the face of the armature plate in ready position a short axial distance, typically a millimeter or two, away from the face of the pulley. The springs are flexed out of that free state when the coil is energized to pull the armature into pulley engagement, and snap back to return the armature to ready position when the coil is deenergized. The leaf springs are oriented at an angle to a chord of the armature plate circle, and act in compression to transfer torque from the armature plate to the drive plate, pushing it, if effect, rather than dragging it. While simple, robust and durable, the basic parts of this mechanism provide no noise cushioning or vibration dampening. Simple rubber bumpers can be added through the drive plate, facing the armature plate and acting independently of the springs, to cushion the noise of disengagement, but these still provide no significant vibration dampening.
One early and simple modification to the basic system was a ring of rubber located intermediate the drive plate and the hub, as seen in U.S. Pat. No. 3,205,989. All torque was transferred through the ring, so that vibrations were well isolated, but rubber is not robust or durable as a torque transfer means, compared to tight metal-to-metal contact, such as riveted springs. Other designs eliminate the metal springs entirely, with several large and discrete rubber elements or “eyes” that are pulled axially out and retract to provide the return spring function, and which are compressed normal to the axial direction to provide dampening. An example may be seen in U.S. Pat. No. 5,184,705. Typically, an enlarged metal rivet fixed to the armature plate extends axially through matching holes in the drive plate and armature, surrounded radially by a rubber member that is either inserted or molded in place. Such designs have similar or worse durability issues than a continuous rubber ring, since the rubber is doing double duty, serving as both axial return spring and torsional damper.
A recent compromise simply combines the standard leaf spring design, with no damping provision, with an independent plurality of discrete, rubber “eye” type dampers. One example may be seen in U.S. Pat. No. 5,667,050. As disclosed there, three conventional leaf springs interconnect the armature and drive plate, as do three independent “eye” type dampers. The design seeks to improve the durability of the rubber in the damper by breaking it into two discrete parts, a flanged rubber washer that is radially contained between the interconnecting rivet and the drive plate through hole, and a separate rubber washer contained between the rivet head and the surface of the drive plate. Such a combination design, in general, is complex and expensive, having three interconnecting leaf springs and three interconnecting dampers, as opposed to just one or the other. In addition, all known designs for dampers interconnecting the drive plate and armature plate have a rubber element radially contained in the space between the rivet and through hole, which is therefore subject to a repeated pinching or sheering stress as torsional oscillations occur. This has serious implications for the durability of the rubber material.